In the production of concrete pipes in upright position in existing pipe moulding machines comprising an elastically supported outer mould and a horizontally vibrated inner mould or core, it often depends on chance whether or not the pipes obtain the desired quality as relates mechanical strength, watertightness and surface finish. More particularly, deficiencies in the form of lack of tightness may appear in the upper end portion of the pipe being moulded, and in such case a closer examination of the defective pipes usually discloses stone accumulations, also known as stone nests, which indicate that the vibration in the zone or zones in question has caused separation of the moulding materials, rather than the intended packing or compacting thereof. As indicated, the deficiencies cannot be characterized as systematic because the majority of the manufactured pipes will normally satisfy the quality requirement and, therefore, there has been considerable uncertainty on the real cause of the moulding deficiencies. It has further been ascertained that a given machine may operate satisfactorily with equipment for some nominal pipe diameters but results in a relative high percentage of scrap with equipment for other diameters, in particular smaller diameters, which has not either been explicable in a satisfactory way.
By the pipe making, fresh concrete is poured into the cavity between the outer mould and the core after the vibration of the core has started. The vibration depends on forces which through upper and lower vibrator bearings are transmitted to the core and which rotate in horizontal planes so as to cause a circular movement of the core. Equipment which operates in this manner is commonly used in the pipe moulding art and is shown, for example, in U.S. Pat. Nos. 3,177,554 and 3,201,843. Eccentric elements on a vertical axis of rotation are positioned within and connected through bearings to the inside of the core moulding member. Rotation of the eccentric elements effects a rotational vibration of the core member, i.e. the core does not rotate about its axis, but each point on the core rotates in a small horizontal circle, following the eccentric vibration of the eccentric elements. Via the poured concrete, and to a certain degree also by other ways, the vibration forces are transmitted onto the outer mould which is therefore also subjected to a circular motion. The frequency of this motion is similar to that of the core, whereas the motions differ as relates their amplitude, i.e. the radii of the circular motions, and their phase. Therefore, a relative movement will occur between the outer mould and the core, which is mainly a radially directed movement but also includes a tangential component. Accordingly, the moulding mass is subjected to oscillating pressure and pull forces intended to cause a rearrangement and packing or compacting of the material, and at the same time the tangential movement may result in a displacement between the moulding mass and the cavity walls.
For obtaining a satisfactory packing, the concrete mass being vibrated must be subjected to a certain power or inertia and, at the same time, a static pressure should be exerted on the mass so as to secure that the vibration does result in the desired packing rather than in a simple pulsating movement that can lead to a separation of the components of the mass and the formation of the stone nests referred to in the foregoing. In the lower regions of the mould, e.g. more than 20 to 30 cm below the concrete level, the weight of the superposed material may be sufficient to provide the required static pressure, whereas the vibration forces in the upper region of the mould may cause a loosening of the material rather than a compacting thereof. This tendency becomes more marked with increasing vibration power, which could point to a reduction of the vibration effect which, however, does not lead to a satisfactory solution because the reduced pressure level is accompanied by a poorer quality. Instead thereof, in the last phase of the moulding operation a pressing and smoothing head has been used which is pressed down into the upper end of the moulding cavity to cause an ironing of the concrete surface and to exert a static pressure thereon. For this purpose it is necessary that the pressing and smoothing head fits almost like an annular piston in the cavity and, accordingly, will form a rather rigid connection between the outer mould and the core so that, generally, these elements will move together. This means that the relative movement between the outer mould and the core and, therefore, the dynamic influence on the moulding mass is drastically reduced. The remaining (static) pressure will cause only a poor compacting, and the concrete quality in the upper portion of the moulded pipe becomes correspondingly poor.
The reduced quality resulting from the less effective compacting would be of minor importance if it were not so that the tendency of material separation and, thus, the formation of the stone nests is very marked in exactly the same portion of the pipe. The separation tendency cannot be eliminated by an increase of the static pressure which does not result in a substantial rearrangement of the material components.